GUN 870/Shotty Crew v.halp!

I'm trying to install the Surefire fore end on my Remington 870. I have the synthetic 7 round version pictured below. I can't for the life of me loosen the magazine cap to disassemble the weapon. It's not just me...I've used pliers padded with towels and the damn thing just won't budge...made a scratch in the finish while trying that move. Is there a trick I just don't know about?

1. Take off barrel clamp
2. Unscrew mag extension by hand
3. Wrap towel around magazine nut
4. Twist off with Channel-lock plyers (nut has locking grooves underneath so it'll sort of click the first few turns)